Retroviruses have been isolated from a variety of murine and primate tissues. The type-D retrovirus, Mason-Pfizer virus (MPV), was isolated from a rhesus monkey mammary carcinoma. Molecular hybridization studies demonstrated that MPV is not present in rhesus monkeys as an endogenous provirus; the complete MPV RNA genome, however, has been found in several animals captured in the wild. These findings demonstrate that MPV is indigenous to the rhesus population and therefore, must be transmitted via a nongerm line mechanism. Radioimmunoassays have been established for group and interspecies antigenic determinants of the internal viral polypeptides of MPV and two other type-D retroviruses: the langur virus and the squirrel monkey retrovirus. The RNA-dependent DNA polymerase (RDDP) of MPV has also been purified to homogeneity. It has been characterized as to its ability to copy synthetic and heteropolymeric templates. The endogenous RDDP of the type-B murine mammary tumor virus has, for the first time, been used to generate a cDNA that is equally representative of the entire MMTV RNA genome. This cDNA is being used to study the expression of specific subsets of nucleic acid sequences in high and low mammary tumor incidence strain of mice.